Integrated liquid storage

ABSTRACT

A system and process for liquefying a gas, comprising introducing a feed stream into a liquefier comprising at least a warm expander and a cold expander; compressing the feed stream in the liquefier to a pressure greater than its critical pressure and cooling the compressed feed stream to a temperature below its critical temperature to form a high pressure dense-phase stream; removing the high pressure dense-phase stream from the liquefier, reducing the pressure of the high pressure dense-phase stream in an expansion device to form a resultant two-phase stream and then directly introducing the resultant two-phase stream into a storage tank; and combining a flash portion of the resultant two-phase stream with a boil-off vapor from a liquid in the storage tank to form a combined vapor stream, wherein the temperature of the high pressure dense-phase stream is lower than the temperature of a discharge stream of the cold expander.

BACKGROUND

Nitrogen liquefiers are well known in the art and are generally linkedto a nitrogen generator, for example, or an Air Separation Unit (ASU).Liquefiers may be used to liquefy low pressure gaseous nitrogen from anASU, for example. Liquefiers may also take at least a part of their feedfrom the ASU at higher pressure and/or at cryogenic temperatures forliquefying purposes.

In traditional liquefaction processes, high pressure nitrogen is cooledto cryogenic temperatures to form a dense-phase fluid (i.e., a fluidbelow its critical temperature and above its critical pressure) and thenreduced in pressure, normally through the use of a valve or dense fluidexpander, so that it forms mostly liquid with some flash vapor. Thistwo-phase mixture is then fed to a separator. A cold expander alsotypically discharges a vapor or slightly liquefied stream into theseparator. Vapor from the separator is re-warmed to ambient temperatureand then recycled in the process, whilst the liquid is subcooled beforebeing fed to an insulated liquid storage tank, for example. Thissubcooling may take place by pressure reduction in a second separator ata lower pressure or indirectly in a subcooler by heat exchange against aboiling liquid at low pressure. The use of a subcooler allows enoughpressure to be maintained in the liquid to transfer it to storagewithout using pumps, for example.

Portions of the liquid produced in the liquefier may be stored, forexample, in an insulated liquid storage tank for future use or beexported by road tanker while other portions of the liquid may bereturned to the ASU to provide refrigeration, for example.

If a second separator is used, the second separator must be elevatedabove the level of the storage tank if the use of additional pumps is tobe avoided.

Storage of the liquid in insulated liquid storage tanks is, however, nota simple solution. Heat ultimately leaks into the insulated liquidstorage tank from the surroundings due to imperfect insulation, forexample. Also, part of the liquid stored in the insulated liquid storagetanks evaporates and requires the production of additional liquid tocompensate for such loss. Traditionally, the cold vapor that is formedas a result of the liquid evaporating in the insulated liquid storagetank is vented to atmosphere to avoid the pressure of the insulatedliquid storage tank from rising, however, refrigeration is then lost inthe process.

Previously disclosed nitrogen liquefiers linked to ASU plants were,therefore, problematic for several reasons. First, recovery of flash orcold boil-off vapor from the insulated liquid nitrogen tank required useof a cold blower. Cold blowers were used to pressurize the flash or coldboil-off vapor from the tank so that it was at sufficient pressure to besent back to the liquefier or ASU to allow for its refrigeration to berecovered. Only part of the refrigeration can be recovered, however,when a blower is used because the blower's power is ultimately added tothe cold stream of boil-off vapor as heat. Moreover, blowers areinconvenient and expensive to install and maintain, and add furthercomplexity to these systems and processes, thus, making use of blowersuneconomical

Second, use of cold end liquid nitrogen separators add complexity to theprocess, and make it more costly to implement as they must all beenclosed within an insulated cold box. Large and complex cold boxes aredifficult to deal with when scheduling shipping routes because certaindestination locations may be hard or even impossible to reach with suchlarge pre-insulated loads (i.e., cold box loads).

Third, liquefaction processes have typically included subcoolers toreduce the flash gas formed in the tank. Such subcoolers also addundesirable cost and complexity to the process.

Moreover, while early liquefiers (i.e., liquefiers used prior to theliquefiers traditionally used today) employed a single expander andutilized only a single separator device, these early liquefiers wererelatively inefficient. To increase the efficiency of the liquefiers,later liquefier designs used multiple expanders and multiple separatorsto recover flash vapors at intermediate pressures. Recovery of the flashvapors at intermediate pressures was thought, for many years and to thisvery day, to be necessary because flash vapor formed as a result of aliquid product entering a liquid storage tank was not desirable and,thus, would have been vented to the atmosphere to control the pressureof the storage tank. Such venting would, of course, result in loss ofthe valuable refrigeration from the flash vapor.

Thus, there was a need in the industrial gases industry for a simple andlow cost liquefaction process with the efficiency benefit of tank flashand boil-off vapor recovery without the complexity of cold blowers, coldend separators, or subcoolers.

SUMMARY

The described embodiments satisfy the need in the art by providing asimplified and efficient liquefier using a liquid storage tank as aflash separator and recovering the flash and boil-off vapor from storagethrough the liquefier. Separators and subcoolers may be eliminated fromthe liquefier design and process. As the cold portion of the liquefieris essentially only a heat exchanger and piping, it may be insulateddirectly and the separate cold box structure eliminated. The describedembodiments utilize a design and process that is opposed to conventionalwisdom for the construction of efficient liquefier designs andprocesses.

Production of liquid in a separate liquefier rather than in an ASU planthas operational advantages such as being easy to turn on and offaccording to demand, but has the significant disadvantages of the highcapital cost and lower efficiency associated with a separate processunit. In general, increasing process efficiency will increase capitalcost, and capital cost has to be increased to improve efficiency. Theprocess and system described allows this capital cost to be reduced atthe same time as improving the efficiency.

In one embodiment, a process for liquefying a gas is disclosed,comprising introducing a feed stream into a liquefier comprising atleast a warm expander and a cold expander; compressing the feed streamin the liquefier to a pressure greater than its critical pressure andcooling the compressed feed stream to a temperature below its criticaltemperature to form a high pressure dense-phase stream; removing thehigh pressure dense-phase stream from the liquefier and reducing thepressure of the high pressure dense-phase stream in an expansion deviceto form a resultant two-phase stream and then directly introducing theresultant two-phase stream into a storage tank; and combining a flashportion of the resultant two-phase stream with a boil-off vapor from aliquid in the storage tank to form a combined vapor stream, wherein thetemperature of the high pressure dense-phase stream is lower than thetemperature of a discharge stream of the cold expander.

In another embodiment, a system for liquefying an atmospheric gas isdisclosed, comprising: a first conduit for accepting a feed stream; aliquefier fluidly connected to the first conduit for compressing andcooling the feed stream to form a high pressure dense phase fluid,wherein the liquefier comprises at least a warm expander, a coldexpander, a compressor for compressing the feed stream to a pressuregreater than its critical pressure, and a heat exchanger, for coolingthe compressed feed stream to a temperature below its criticaltemperature; a second conduit fluidly connected to the liquefier foraccepting the high pressure dense-phase stream from the liquefier; afirst expansion device fluidly connected to the second conduit to reducethe pressure of the high pressure dense-phase stream to form a resultanttwo-phase stream; a third conduit fluidly connected to the firstexpansion device for accepting the two-phase expanded stream; and astorage tank fluidly connected to the third conduit for accepting andstoring the two-phase expanded stream, wherein the storage tank isdesigned to operate at a pressure at or below 1.5 bara, and wherein theheat exchanger is designed such that the temperature of the highpressure dense-phase stream is lower than the temperature of a dischargestream of the cold expander.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofexemplary embodiments, is better understood when read in conjunctionwith the appended drawings. For the purpose of illustrating embodiments,there is shown in the drawings exemplary constructions; however, theinvention is not limited to the specific methods and instrumentalitiesdisclosed. In the drawings:

FIG. 1 is a flow diagram of an exemplary process for using a liquidstorage tank as a flash separator and recovering the flash and boil-offvapor from storage through the liquefier, in accordance with the presentinvention;

FIG. 2 is a flow diagram of an alternative exemplary processincorporating a different liquefier configuration;

FIG. 3 is a flow diagram of a previously disclosed process with the sameexpander configuration as shown in FIG. 1, wherein the process includesa cold end separator and subcooler, but comprises no flash vapor orboil-off recovery from the tank; and

FIG. 4 is a flow diagram illustrating various ways to integrate theexemplary process of FIG. 1 with an Air Separation Unit where any otherprocess according to the invention may be integrated with the AirSeparation Unit in a similar fashion.

DETAILED DESCRIPTION

FIG. 1 illustrates an exemplary system and process for using a liquidstorage tank 170 as a flash separator and recovering the flash andboil-off vapor from the liquid storage tank 170 through the liquefier101. FIG. 1 discloses low pressure nitrogen feed stream 100 beingcombined with warmed tank flash and boil-off vapor stream 102 to formcombined stream 104. The low pressure feed stream 100 may be nitrogen,or it may be another gas or gas mixture such as air, oxygen, argon,carbon monoxide, neon, ethylene, helium, or hydrogen, for example. Thecombined stream 104 is then compressed in the feed compressor 106 toabout 6 bara to form compressed stream 108. Compressed stream 108 isthen cooled in an aftercooler 110 to form cooled stream 112. Cooledstream 112 is then combined with recycle stream 114 to form stream 116.Stream 116 is then compressed in recycle compressor 118 to about 32 bararesulting in compressed stream 120. Stream 120 is then cooled in anaftercooler 122 to form stream 124. Stream 124 is then split intostreams 126 and 128.

Stream 126 is (optionally) cooled in the heat exchanger 130 to formstream 132. Stream 132 is then expanded in warm expander 134 to around 6bara to form warm expanded stream 136.

Stream 128 is further compressed in the warm compander compressor 138 toform stream 140. Stream 140 is then cooled in the warm companderaftercooler 142 to form cooled stream 144. Cooled stream 144 is thencompressed again in cold compander compressor 146 to about 65 bara toform compressed stream 148. Compressed stream 148 is then cooled againin the cold compander compressor aftercooler 150 to form high pressurestream 152. This high pressure stream 152 is cooled in the heatexchanger 130 to an intermediate temperature of about 182 K, producingstreams 154 and 156.

Stream 156 is expanded in a cold expander 158 to form discharge stream160. Discharge stream 160 is returned to the cold end of the heatexchanger 130 where it is warmed and mixed with the exhaust stream 136from the warm expander 134 to form stream 162. Stream 162 is warmed inheat exchanger 130 to form recycle stream 114. Recycle stream 114 isthen mixed with compressed feed stream 112 and fed to the suction of therecycle compressor 118.

Stream 154 is further cooled in the heat exchanger 130 to form a highpressure dense-phase stream 164. High pressure dense-phase stream 164 iswithdrawn from the cold end of the heat exchanger 130 at a temperatureof about 96 K, reduced in pressure across one or more expansion devices166 to form stream 168, where stream 168 is fed directly into a liquidstorage tank 170. As used herein, the term “fed directly” shall meanthat the designated stream, after exiting the one or more expansiondevices 166 is provided to the liquid storage tank 170 via a conduitwithout encountering any further apparatus that would alter thecomposition, temperature, or pressure of the designated stream.Moreover, as used herein “directly connected” shall mean that a firstdevice or piece of an apparatus is connected to a second device or pieceof an apparatus without any intermediate devices or pieces of apparatusthat would alter the composition, temperature, or pressure of a streampassing through, for example, the first device to the second device.

Stream 168 is flashed into the liquid storage tank 170 to produce mostlyliquid with some vapor. The liquid from stream 168 will add to theliquid already present in the liquid storage tank 170, whilst the flashvapor will combine with boil-off vapor already present in the liquidstorage tank 170. A combined vapor stream 172 composed of flash vaporand boil-off vapor is withdrawn from the liquid storage tank 170, and,during normal operation, is fed to the heat exchanger 130 of theliquefier 101 as stream 174. Stream 174 is warmed in the heat exchanger130 to form warmed tank flash and boil-off vapor stream 102 and mixedwith the low pressure feed 100 to form combined stream 104 entering themake-up compressor 106 of the liquefier 101.

If the liquefier 101 is not operating, the liquid storage tank 170boil-off vapors can be removed from the liquid storage tank 170 ascombined vapor stream 172, 176, reduced in pressure across one or moreexpansion devices 178 to form stream 180, and vented to the atmosphereto control the pressure of the liquid storage tank 170.

One of the significant benefits of this system arrangement is thesimplified design. Heat exchanger 130, expanders 134, 158 and theassociated piping may be insulated separately, for example, with aninsulating material such as mineral wool, polyurethane foam, foamglass,“cryogel,” or a suitable alternative, or installed in small local coldboxes connected by insulated piping. Reducing the size requirements ofthe cold box is especially important when dealing with and schedulingshipping routes because certain destination locations may be hard orimpossible to reach with larger pre-insulated loads (i.e., cold boxloads).

Further, contrary to traditional belief, recovery of the boil-off vaporfrom the liquid storage tank 170 surprisingly improves the overallefficiency of the liquefier 101 and storage system by around 0.5-1.0%(depending on the relative sizes of the liquid storage tank 170 andliquefier 101 and the quality of tank insulation) compared to previousdesigns where the boil-off gas was not recovered, as its cold is used topartially cool the product and reduce the power required by theliquefier 101 rather than being wasted by venting it directly toatmosphere. In addition, the required nitrogen feed flow is reduced (asthe previously vented nitrogen is recovered) which could lead to use ofsmaller ASUs.

If the low pressure nitrogen feed stream 100 to the liquefier 101 is ata pressure high enough to provide the low pressure nitrogen feed stream100 directly into the suction of the recycle compressor 118, the feedcompressor 106 may also be eliminated, and in that case, the warmed tankflash and boil-off vapor stream 102 may be vented to the atmospherethrough a valve to simply control the pressure of the liquid storagetank 170.

With surprising and unexpected result, Applicants found that if highpressure dense-phase stream 164 is cooled below the temperature ofdischarge stream 160 through indirect heat exchange against therecovered combined vapor stream 174 in heat exchanger 130, thenreduction of the pressure of high pressure dense-phase stream 164 to thepressure of discharge stream 160 would not result in the generation ofsignificant amounts of flash vapor, thus, the efficiency of theliquefier 101 is not reduced by eliminating the additional separator andits related components. In fact, one skilled in the art will appreciatethat this exemplary embodiment eliminates the need for separators andsubcoolers (for example separator 304 and subcooler 310 of FIG. 3) whilemaintaining a high level of efficiency. For example, while traditionalsystems and processes may have used two or more separators to recoverthe flash vapors at high and reduced pressures, the disclosed system andprocess achieves the same result minus substantial capital cost andsubstantial transport planning while achieving equal or betterefficiencies.

In another embodiment, and as illustrated in FIG. 2, a similar systemand process to FIG. 1 is disclosed; however this embodiment comprises adifferent expander arrangement. In this system/process, stream 124 fromthe recycle compressor aftercooler 122 is split into two streams 226 and228 that feed the compressor ends of the warm and cold companders 238and 246 arranged in parallel. The respective outlet streams 240 and 248of the warm and cold companders 238 and 246 are combined into stream 249and cooled in aftercooler 250 before being fed to heat exchanger 130 asstream 252. Stream 252 is cooled to a first intermediate temperature inheat exchanger 130 before being split into streams 232 and 253.

Stream 232 is expanded in warm expander 234 to form stream 236 andcombined with warming discharge stream 160 forming stream 162 at anintermediate location of the heat exchanger 130. Stream 253 is furthercooled to a second intermediate temperature and split again into streams256, 254. Stream 256 is expanded in cold expander 258 to form dischargestream 160. Discharge stream 160 is then warmed in the heat exchanger130. Stream 254 is further cooled in heat exchanger 130 to form the highpressure dense-phase stream 164 that is fed to the liquid storage tank170 via expansion device 166.

FIG. 3 is a flow diagram of a previously disclosed prior art processwith the same expander configuration as shown in FIG. 1 but where theprocess comprises no flash vapor or boil-off recovery from the tank.FIG. 3 is provided for exemplary purposes and to be used to compare withthe system and process of FIG. 1.

As illustrated in FIG. 3, a cold end separator 304 and subcooler 310 areincorporated in the liquefier 301 and there is no recovery of the flashor boil-off vapor from the liquid storage tank 170. The high pressuredense-phase stream 164 from the cold end of the heat exchanger 130 isreduced in pressure in one or more expansion devices 300 and theresulting two-phase stream 302 is then fed to a separator 304 along withthe cold expander discharge stream 160 that may contain some liquid.Vapor stream 306 from separator 304 is warmed in heat exchanger 130 toan intermediate temperature where it is combined with the warm expanderexhaust stream 136 to form stream 162. Liquid stream 308 from separator304 is subcooled in subcooler 310 to about 78 K to form stream 312. Aportion 316 of subcooled liquid stream 312 is reduced in pressure in oneor more expansion devices 318 and then evaporated in subcooler 310 toform vapor stream 320 and reheated in heat exchanger 130 to form stream102. The remaining portion 314 of subcooled liquid stream 312 is fed tothe liquid storage tank 170 via one or more expansion devices 166 toform stream 168 where stream 168 is fed into the liquid storage tank170. Flash and boil-off vapor from the liquid storage tank 170 is ventedvia stream 176 through expansion device 178 to form stream 180 (to bevented to the atmosphere) to control the tank pressure.

FIG. 4 is a flow diagram illustrating several exemplary options forintegrating the liquefier system and process of FIG. 1 with an ASU ornitrogen generator. For example, the low pressure nitrogen feed stream100 from the warm end of the ASU may be completely or partly replaced byone or more of alternative feed streams 400, 404, or 408.

A high pressure nitrogen stream 400 from the warm end of the ASU ornitrogen generator may also be mixed with stream 112 from the feedcompressor aftercooler 110 to form stream 402 that may then be mixedwith stream 114 to form stream 116 that is fed to the recycle compressor118. Alternatively, stream 400 may be mixed downstream of where stream114 combined with stream 112, or into an interstage location of the feedcompressor 106 or recycle compressor 118.

A low pressure nitrogen stream 404 from a low pressure column orsubcooler at the cold end of the ASU may be mixed with the returning lowpressure stream 174 from the liquid storage tank 170 to form stream 406that is then heated in the heat exchanger 130.

A cold high pressure nitrogen stream 408 from a high pressure column ofthe ASU or nitrogen generator or the single column of a single columnnitrogen generator may be mixed with the discharge stream 160 from thecold expander 158 to form stream 410 that is then heated in heatexchanger 130.

Additionally, a divided portion stream 412 of the high pressuredense-phase stream 164 from the cold end of the liquefier may be feddirectly to the ASU or nitrogen generator to provide refrigerationwhilst the remaining portion 414 may be fed to the liquid storage tank170. As used herein a “divided portion” of a stream shall mean a portionhaving the same chemical composition as the stream from which it wastaken. Divided portion stream 412 may be fed, for example, to the HighPressure (HP) column, the Low Pressure (LP) column, the subcooler, orthe heat exchanger of an ASU.

EXAMPLE

Tables 1 and 2 provide exemplary flow rates, temperatures, and pressuresfor the configurations/processes of FIG. 1 and FIG. 3. Theconfiguration/process disclosed in FIG. 1 resulted in the data of Table1, where 300 tonnes per day of liquid nitrogen was produced in liquidstorage tank 170. The configuration/process consumed approximately 5950kW of electricity.

TABLE 1 Stream 100 102 114 132 152 156 160 164 174 in tank Flow 446 1222386 978 1977 1409 1409 569 122 446 (kmol/hr) Temperature 299 299 299267 303 182 97 96 78 78 (K) Pressure 1.03 1.03 6.00 31.84 64.80 64.606.20 64.60 1.10 1.10 (bar (abs))

The configuration/process disclosed in FIG. 3 resulted in the data ofTable 2, where 300 tonnes per day of liquid nitrogen was also producedin liquid storage tank 170. This configuration/process consumedapproximately 6000 kW of electricity.

TABLE 2 Stream 100 102 114 132 152 156 160 164 176 312 314 in tank Flow460 97 2552 1238 1871 1369 1369 502 13 557 460 446 (kmol/hr) Temperature299 299 299 254 303 174 97 99 78 79 79 78 (K) Pressure 1.03 1.03 6.0030.05 64.80 64.60 6.20 64.60 1.10 6.00 6.00 1.10 (bar (abs))

Importantly, the exemplary process of FIG. 1/Table 1 produces the samenet quantity (446 kmol/hr) of liquid nitrogen in the liquid storagetank, but uses 0.8% less power than the previously disclosed process ofFIG. 3/Table 2, has a 3% lower feed rate (stream 100) due to therecovery of flash and boiloff vapor from the liquid storage tank (stream174) and elimination of tank boil-off losses to atmosphere (stream 176),and provides significant capital cost savings from the elimination of afirst separator, a second separator or subcooler, and their associatedvalves, controls and insulating enclosure. As the cold portion of theliquefier comprises essentially only a heat exchanger and the associatedpiping, the liquefier equipment may be insulated directly and theseparate cold box structure required to contain and insulate the firstseparator, the second separator or subcooler, and their associatedvalves, and controls may be eliminated, thus, significantly reducing thesize of the cold box. Reducing the size requirements of the cold box isespecially important when dealing with and scheduling shipping routesbecause certain destination locations may be hard or even impossible toreach with larger pre-insulated loads (i.e., cold box loads).

While aspects of the present invention has been described in connectionwith the preferred embodiments of the various figures, it is to beunderstood that other similar embodiments may be used or modificationsand additions may be made to the described embodiment for performing thesame function of the present invention without deviating therefrom.Therefore, the claimed invention should not be limited to any singleembodiment, but rather should be construed in breadth and scope inaccordance with the appended claims.

1. A process for liquefying a gas, comprising: introducing a feed streaminto a liquefier comprising at least a warm expander and a coldexpander; compressing the feed stream in the liquefier to a pressuregreater than its critical pressure and cooling the compressed feedstream to a temperature below its critical temperature to form a highpressure dense-phase stream; removing the high pressure dense-phasestream from the liquefier and reducing the pressure of the high pressuredense-phase stream in an expansion device to form a resultant two-phasestream and then directly introducing the resultant two-phase stream intoa storage tank; and combining a flash portion of the resultant two-phasestream with a boil-off vapor from a liquid in the storage tank to form acombined vapor stream, wherein the temperature of the high pressuredense-phase stream is lower than the temperature of a discharge streamof the cold expander.
 2. The process of claim 1, further comprisingheating at least part of the combined vapor stream to ambienttemperature.
 3. The process of claim 2, further comprising mixing thewarmed combined vapor stream with the feed stream for recycle.
 4. Theprocess of claim 2, further comprising venting the warmed combined vaporstream to the atmosphere to control the pressure of the storage tank. 5.The process of claim 2, wherein the pressure of the storage tank is lessthan 1.5 bara.
 6. The process of claim 1, further comprising removing atleast part of the combined vapor stream from the storage tank, reducingthe pressure of the combined vapor stream in one or more expansiondevices to form a low pressure combined vapor stream, and venting thelow pressure combined vapor stream to the atmosphere to control thepressure of the storage tank.
 7. The process of claim 1, wherein thefeed stream is a low pressure nitrogen feed stream from a warm end of anAir Separation Unit.
 8. The process of claim 1, further comprisingmixing a low pressure nitrogen stream from a low pressure column orsubcooler of an Air Separation Unit with the combined vapor stream fromthe storage tank prior to heating.
 9. The process of claim 1, furthercomprising taking a divided portion of the high pressure dense phasefluid from the liquefier, feeding the divided portion of the highpressure dense phase fluid directly to an Air Separation Unit ornitrogen generator to provide refrigeration.
 10. The process of claim 9,wherein the divided portion of the high pressure dense phase fluid isreduced in pressure and fed to a High Pressure (HP) column, a LowPressure (LP) column, a subcooler, or a main heat exchanger of the AirSeparation Unit.
 11. A system for liquefying an atmospheric gas,comprising: a first conduit for accepting a feed stream; a liquefierfluidly connected to the first conduit for compressing and cooling thefeed stream to form a high pressure dense phase fluid, wherein theliquefier comprises at least a warm expander, a cold expander, acompressor for compressing the feed stream to a pressure greater thanits critical pressure, and a heat exchanger, for cooling the compressedfeed stream to a temperature below its critical temperature; a secondconduit fluidly connected to the liquefier for accepting the highpressure dense-phase stream from the liquefier; a first expansion devicefluidly connected to the second conduit to reduce the pressure of thehigh pressure dense-phase stream to form a resultant two-phase stream; athird conduit fluidly connected to the first expansion device foraccepting the two-phase expanded stream; and a storage tank fluidlyconnected to the third conduit for accepting and storing the two-phaseexpanded stream, wherein the storage tank is designed to operate at apressure at or below 1.5 bara, and wherein the heat exchanger isdesigned such that the temperature of the high pressure dense-phasestream is lower than the temperature of a discharge stream of the coldexpander.
 12. The system of claim 11, wherein the storage tank isdirectly connected to the third conduit and wherein the first expansiondevice is directly connected to the second conduit.
 13. The system ofclaim 11, further comprising a fourth conduit fluidly connected to thestorage tank for accepting a combined vapor stream comprising a flashvapor portion of the resultant two-phase stream and a boil-off vaporportion from a liquid in the storage tank.
 14. The system of claim 13,wherein the fourth conduit is fluidly connected to the heat exchangerand the first conduit.
 15. The system of claim 13, further comprising asecond expansion device fluidly connected to the fourth conduit toreduce the pressure of the combined vapor stream to control the pressureof the storage tank.